Aarne Mämmelä

Performance improvement with predictive channel selection for cognitive radios

Prediction of future availability times of different channels based on history information helps a cognitive radio (CR) to select the best channels for control and data transmission. Different prediction rules apply to periodic and stochastic ON-OFF patterns. A CR can learn the patterns in different channels over time. We propose a simple classification and learning method to detect the pattern type and to gather the needed information for intelligent channel selection. Matlab simulations show that the proposed method outperforms opportunistic random channel selection both with stochastic and periodic channel patterns. The amount of channel switches needed over time reduces up to 55%, which reduces also the delay and increases the throughput.

Classification-Based Predictive Channel Selection for Cognitive Radios

The proposed method classifies traffic patterns of primary channels in cognitive radio systems and applies different prediction rules to different types of traffic. This allows a more accurate prediction of the idle times of primary channels. An intelligent channel selection scheme then uses the prediction results to find the channels with the longest idle times for secondary use. We tested the method with Pareto and exponentially distributed stochastic traffic and with deterministic traffic. The predictive method using past information improves the throughput of the system compared to a system based on instantaneous idle time information. The classification-based predictive method improves the performance compared to pure prediction when the channels of interest include both stochastic and deterministic traffic. The amount of collisions with a primary user can drop 60 % within a given interval compared to a predictive system operating without classification.

Clock-Gating in FPGAs: A Novel and Comparative Evaluation

Clock-gating has been employed in low-power FPGA designs based on an emulated and compromised method. So far in literature the actual efficiency of savings in power consumption is not thoroughly studied for this method. In this paper we evaluated the clock-gating technique in FPGAs, based on a novel and comparative process. For a set of design cases, both the FPGA and ASIC clock-gating methods were implemented. Figures of power consumption were obtained using the corresponding FPGA/ASIC power estimation tools and devices. The results show that in FPGAs, the efficiency of savings in dynamic power consumption is about 50% to 80% of its ASIC counterparts. However, we also found that compared to ASICs, clock-gating for FPGAs in terms of the efficiency of savings in total average power consumption was only about 6% to 30% of its ASIC counterparts due to FPGAs large static power consumption

Cooperative and noncooperative spectrum sensing techniques using Welch’s periodogram in cognitive radios

Radio spectrum deployment is growing considerably. With this respect finding unutilized frequency channels for new applications has become much more challenging. The problem can be solved by letting unlicensed systems to dynamically use unexploited licensed bands. This kind of flexible spectrum usage requires telecommunication systems to be equipped by an ability to specify unoccupied parts of radio spectrum. One method to identify temporarily unused parts of radio spectrum is spectrum sensing. In this paper, we focus on spectrum sensing using Welchs periodogram. In particular, we generalize and apply the theoretical analysis of the energy detection to the Welchs periodogram. Furthermore, we extend our study to cooperative spectrum sensing. The results indicate that the cooperation between two radios provides the highest cooperation gain.

Wireless channel equalisation

Equalisation techniques for wireless channels, in particular for those encountered in mobile wireless communications, are examined. Equalisation is broadly defined to include reception techniques which estimate the state or response of the channel and then attempt to compensate for its effects. The paper considers equalisation techniques for fading dispersive channels which include both time and frequency selectivity. In addition, brief consideration is given to the problems of blind equalisation, techniques for dealing with fast fading channels and to the problem of joint equalisation and decoding. The paper does not attempt to provide in-depth analysis or performance results. Rather, the interested reader is referred to the extensive list of references.

Cooperative spectrum sensing using quantized soft decision combining

In this paper, a novel method combining cooperative spectrum sensing with quantized soft decision combining is introduced. In order to allow cognitive radios and cognitive networks to opportunistically use spectrum, it is a prerequisite that the license owner or primary user of the spectrum will not be harmfully interfered and the spectrum band will be vacated as soon as the primary user starts its own transmission. There are results indicating that the reliability of sensing information can be improved by exploiting the spatial dimension via cooperation between cognitive radios. Our approach is to further improve the reliability by sharing sensing information between cooperative radios using quantized soft decision combining. Simulations are conducted for the proposed two bit quantized soft decision combining, hard decision combining and nonquantized soft decision combining in an additive white Gaussian noise (AWGN) channel using Welchs periodogram. Hard decision combining is considered with three different decision making rules and the obtained simulation results are verified with analytical performance results for Welchs periodogram. The results show substantial improvement in the detection probability when sensing information between cooperating nodes is shared using two bits instead of one. By using an additional bit it is possible to reach detection probabilities that in hard decision combining would have required one or more additional cooperative users. The results also indicate that the increase in probability of detection is not as significant when full observation of the signal energy is shared between cooperative radios instead of two bits. Thus, almost all the achievable benefit from soft decision combining can be obtained with the proposed quantized soft decision combining.

Comparison of direct learning and indirect learning predistortion architectures

Power amplifiers in a communication system are inherently nonlinear. Digital predistorters can compensate these nonlinearity effects. In this paper, two memory polynomial predistorters including direct and indirect learning architectures are compared with each other. To the best of our knowledge, no similar comparisons have been published. Both of these architectures are special cases of the self-tuning control. We have modeled predistorters and analysed nonlinear effects of a power amplifier and their digital compensation by using Matlab¿. Simulation results show that the memory polynomial model has convergence problems at large amplitudes and also problems of accuracy of representation. We observed that the results of the compensation depend also on the amplitude, not only on the frequency. The results of the linearisation show that the direct learning architecture achieves a better performance in almost all cases.

Constant envelope multicarrier modulation: performance evaluation AWGN and fading channels

In this paper we study the suitability of constant envelope multi-carrier modulation technique for the implementation of 1 Gbps wireless link at 60 GHz. This technique combines orthogonal frequency division multiplexing (OFDM) and phase modulation (PM) where: (1) PM creates a constant envelope signal which allows high power amplifier to operate near saturation levels thus maximizing power efficiency, (2) OFDM increases robustness to multipath fading. Since OFDM-PM symbols satisfy symmetry property, maximum-ratio combiner (MRC) can be used at the receiver. Our simulations show that in an AWGN channel at bit error level 10-3, the OFDM-PM with MRC has about 0.8 dB performance loss compared to OFDM or single carrier minimum shift keying (MSK). For Rician fading channels, we find that OFDM-PM performs comparably to MSK and outperforms uncoded OFDM. Furthermore, we show that both OFDM and OFDM-PM have similar bit error distribution characteristics, and thus the performance of OFDM-PM can be improved by the use of water-filling or coding techniques

Spectrum Occupancy Measurements: A Survey and Use of Interference Maps

In order to provide meaningful data about spectrum use, occupancy measurements describing the utilization rate of a specific frequency band should be conducted over a specific area instead of a single location. This paper presents a comprehensive methodology for the measurement and analysis of spectrum occupancy. This paper surveys spectrum measurement campaigns and associated interference maps, introducing the latter as a tool for spectrum analysis and management based on measurement data. An interference map characterizes the spectrum use by defining the level of interference over an area of interest in a certain frequency band. Building on findings from practical measurement studies, guidelines for spectrum occupancy measurements are given. While many scientific spectrum occupancy measurement papers tend to be too optimistic about the significance and generality of the results, we propose a cautionary perspective on drawing strong conclusions based on the often limited amount of data gathered. The different phases of the spectrum occupancy measurement and analysis process are described and a thorough discussion of interpolation methods is provided. Means to improve the measurement accuracy are discussed, especially regarding spatial domain considerations and the impact of the sampling interval on the results. A practical example of an improved measurement system design covering all the phases of the measurement process and used at the Turku, Finland; Blacksburg, VA, USA; and Chicago, IL, USA, spectrum observatories is given. Using the improved design, more realistic spectrum occupancy data can be obtained to lay the foundation for spectrum management decisions.

Distributed and directional spectrum occupancy measurements in the 2.4 GHz ISM band

This paper presents distributed and directional spectrum occupancy measurements in the 2.4 GHz ISM band. Spectrum occupancy measurements can be used to assess how efficiently the spectrum bands are used today. Future cognitive radio systems can improve the spectrum occupancy by filling the gaps in the prevailing spectrum by opportunistically using unoccupied channels. Most of the spectrum occupancy measurements in the literature have been conducted by using a single measurement device with an omnidirectional antenna. The resulting spectrum occupancy values have presented an average of the overall situation. To characterize the influence of the spatial dimension on the spectrum occupancy in a given area, we introduce the directional spectrum occupancy metric. Directional spectrum occupancy is defined as the fraction of time that the received power in a channel exceeds a threshold in a given measurement direction. We have used two separately located measurement devices with directional antennas to measure the directional spectrum occupancy in an office area with heavy traffic load. The results indicate that the spectrum occupancy is heavily dependent on the measurement location and direction. The influence of the spatial dimension is therefore very crucial in the development of future cognitive radio systems.